This invention relates to the measurement of magnetic fields and, more particularly, to reducing the background magnetic field in the region space in which the measurement is made.
Biomagnetometer systems have been developed which have the necessary sensitivity to detect and measure the tiny magnetic fields naturally produced by the human brain and other parts of the human neurological system. Such biomagnetometer systems are described, for example, in U.S. Pat. Nos. 5,494,033 and 5,471,985, whose disclosures are incorporated herein by reference. In biomagnetometry, those magnetic fields produced by the body are measured and studied to diagnose disorders of the body as well as to understand the functioning of the body.
The magnetic fields produced by the human body are on the order of 1/10,000,000 of the strength of the background magnetic fields produced by the earth and by the equipment and infrastructure present in all urban environments. A major challenge to those wishing to practice biomagnetometry is to measure the tiny biomagnetic fields produced by the human body in the presence of the much larger and everpresent background magnetic fields. The most common approach to the solution of this problem is to perform the biomagnetic measurement in a magnetically shielded room or enclosure (xe2x80x9cMSRxe2x80x9d). Such MSRs are typically made up of several layers of highly magnetically permeable materials which serve to shield from the interior of the MSR the magnetic fields produced by sources outside the MSR, as well as electrically conductive layers which shield the interior of the MSR from external electromagnetic fields. Examples of such MSRs are described in U.S. Pat. Nos. 5,081,071 and 5,335,464, whose disclosures are incorporated by reference.
Magnetically shielded rooms which provide sufficient shielding to enable practical biomagnetometry on humans must attenuate the magnetic fields produced by sources outside the MSR by at least a factor of 100 and must be effective for both dc magnetic fields and ac magnetic fields at frequencies from about 0.01 Hz to several kiloHertz. As a result of this requirement, suitable MSRs require a large amount of highly magnetically permeable shielding material. Suitable MSRs are both expensive and heavy. Typical MSRs now available weigh about 16,000 pounds and cost about $500,000.
The high cost and large weight of these MSRs have limited the practice of both clinical and research biomagnetometry to those institutions which can afford such a cost and can provide permanent space in a building capable of housing such a structure. This limitation has prompted efforts to develop alternative ways of providing magnetic and electromagnetic shielding for a biomagnetometer that are effective, are of lower cost, are lighter in weight, and are easier to house in typical clinical and research buildings.
One approach to the provision of an alternative shielding method has been active cancellation. In this technique, magnetic field detectors detect the background magnetic field and provide command signals to electrical coils that are driven to produce a magnetic cancellation field that is opposite to the background magnetic fields. This active cancellation reduces the ambient magnetic field in the vicinity of the biomagnetometer, so that greater sensitivity is obtained in the biomagnetic measurements. Another approach has been the use of spatial filtering in which certain weighted sums of the output signals are particularly sensitive to sources very close to the biomagnetometer and much less sensitive to interference signals originating far from the biomagnetometer. Although these techniques have been successful to a degree, they still have not achieved the desired results in terms of providing adequate sensitivity for effective biomagnetometry combined with a reduction in the cost and weight of the MSR. Accordingly, there remains a need for a better approach to improving biomagnetic measurement results, while at the same time reducing the cost and weight of the required shielding. The present invention fulfills this need, and further provides related advantages.
The present invention provides an apparatus and a method for performing magnetic field measurements with a high degree of sensitivity in a typical urban hospital environment. The effects of background magnetic fields are reduced, and the cost, weight, and complexity of the shielding of the MSR are reduced. The present approach is particularly useful when applied in human biomagnetic measurements, which require a relatively large MSR enclosure to accommodate the person being measured.
In accordance with the invention, an apparatus comprises an enclosure having a wall defining an interior of the enclosure. The enclosure is preferably the size of a small room, at least about 500 cubic feet in volume. The wall comprises a layer of a highly magnetically permeable material disposed such that there is no layer of an electrically conductive material located closer to the interior of the enclosure than the layer of the highly magnetically permeable material. A background-field magnetometer, preferably a vector magnetometer, within the enclosure measures a background magnetic field and has a background-field magnetometer output signal. An electrical coil structure within the interior of the enclosure has an electrical coil input and produces a magnetic output field tending to nullify the background magnetic field. A controllable electrical current source has a current source output in communication with the electrical coil input, and a current source command signal input. A background-field-reducing feedback controller has a controller input responsive to the background-field magnetometer output signal, and a controller output in communication with the current source command signal input.
Preferably, the electrical coil structure includes at least three electrical coils arranged such that the output magnetic fields of the respective electrical coils are noncollinear, and wherein the current source output includes a separate current source output connected with each of the respective electrical coils.
The wall of the enclosure preferably comprises a layer of highly magnetically permeable material and a structure for mechanical support of that layer. The wall may also include one or more layers of additional shielding material. One or more of such additional layers may be made from electrically conducting material. But all such additional layers are disposed exteriorly of the layer of highly magnetically permeable material. The layer of highly magnetically permeable material is desirably mu metal.
The apparatus typically further comprises a signal magnetometer, preferably a biomagnetometer, positioned to detect a magnetic field produced by a source within the enclosure. The signal magnetometer may include a superconducting quantum interference device for high sensitivity.
In an active feedback system that partially or totally cancels the background magnetic field, the background-field magnetometer and the cancellation coils that are driven responsive to the signal measured by the background-field magnetometer must both be inside the enclosure of the MSR or both be outside the enclosure of the MSR. Otherwise, phase delays caused by the walls of the MSR prevent effective cancellation of the background magnetic field. In the past, it has been the practice to place both the background-field magnetometer and the cancellation coils outside of the enclosure of the MSR because it was believed that if the cancellation coils were placed inside the enclosure, electromagnetic reflections from the electrically conductive walls of the MSR would produce phase delayed xe2x80x9cimagexe2x80x9d fields that would interfere with the desired cancellation of the background magnetic field. Such an approach has had only limited success, however, because the background-field magnetometer is located far from the signal magnetometer and outside the attenuating effect of the walls of the MSR; this has limited its usefulness to canceling highly uniform background fields.
The present approach places highly magnetically permeable material at the side of the wall of the MSR and requires that there is no sheet of electrically conductive material closer to the interior of the enclosure than is the sheet of the highly magnetically permeable material. It further places the background-field magnetometer and cancellation coils inside the MSR. Owing to the response characteristics of the highly magnetically permeable material comprising the inner surface of the enclosure to the magnetic fields produced by the cancellation coils, any phase delayed xe2x80x9cimagexe2x80x9d fields are reduced so that effective cancellation of the background-field can be performed. The modified wall structure having the innermost layer of the highly magnetically permeable material and the positioning of the background-field magnetometer and the electrical coil structure within the enclosure cooperate to provide the benefits of the present approach. The background-field magnetometer and the electrical coil structure are positioned within the enclosure, thereby benefiting from the field-attenuating effects of the MSR wall, and are close to the signal magnetometer so that the cancellation field is representative of that found at the signal magnetometer.
Another benefit of the present approach is that the improved active magnetic-field cancellation allows the amount of the highly magnetically permeable material to be reduced. In current MSRs, there are typically 2-3 layers of the highly magnetically permeable material provided to achieve the necessary reduction of the internal magnetic field. With the improved magnetic-field cancellation of the present approach, typically only a single layer of the highly magnetically permeable material is required. The cost and weight of the MSR are thereby substantially reduced, both because large sheets of the highly magnetically permeable material are expensive and also because the supporting structure may be significantly reduced.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.